1. Field of the Invention
This invention generally relates to an improved process and apparatus for electrical discharge shaping a workpiece by using switching elements to pass a pulse current having a special slant line waveform which is different from a rectangular waveform across the working gap from the initiation of the discharge so as to perform in an optimum condition.
2. Description of the Prior Art
In processes for the electrical discharge shaping of a workpiece, the state of the working gap was naturally variable, and often resulted in an abnormal discharge across the working gap which would thereby damage the workpiece and the electrode, if the electrical condition, such as the mean value of the discharge current, was left uncontrolled. The operator therefore had to adjust the electrical condition depending upon the state of the working gap. However, it was found to be difficult to adjust the mean value of the discharge current and the operator had to have considerable experience and skill in finding an optimum electirical condition. In an attempt to solve these problems the following prior art was developed.
FIG. 1 is a schematic diagram of a conventional prior art apparatus for shaping a workpiece by electrical discharge. In FIG. 1, a base current is passed to each base of switching transistors 1a, 1b, 1c, . . . , 1n, thereby turning on each of the switching transistors so that the discharge current is passed across the working gap between an electrode 5 and a workpiece 6. The discharge current is provided by a DC power source 3 through the switching transistors 1a, 1b, 1c, . . . , 1n and collector resistors 4a, 4b, 4c, . . . , 4n of the switching transistors.
FIGS. 2a and 2b show voltage and current waveforms appearing across the working gap of the apparatus in FIG. 1. In FIGS. 2a and 2b, a reference numeral 7 designates a pulse width; 8 a quiescent time interval; 9 a non-load voltage time interval; 10 a discharge time interval; 11 a non-load voltage; 12 a discharge voltage; 13 a discharge current; 14 a discharge current peak value; and 15 a mean processing current. Under stable processing conditions, the non-load voltage 11 appears with a high probability, and its mean time interval 9 is controlled to be constant by a servo mechanism capable of maintaining constant the mean processing voltage across the working gap. However, this control is stable only when the state of the working gap is good. In other words, if the state of the working gap has deteriorated due to, for example, a deposit of powders between the working gap, then the non-load voltage time interval diminishes or even vanishes. As a result thereof, it is very likely that the discharge will be concentrated at a specific point, and thus will produce a hollow portion on the workpiece. Under such conditions, the mean processing current will increase as compared to that of FIGS. 2(b). If this condition continues for a certain time interval, extinction of ions across the working gap will not properly occur, and the discharge concentrated at one point on the workpiece will continue and the state of the working gap will be further impaired. To solve this problem, the state of the working gap must be restored by decreasing the mean current.
One prior art method of decreasing the mean current was to detect the state of the working gap in terms of discharge current mean value and to change the oscillation frequency according to the detected value. Although this method imparted quick response to the varying state of the working gap, it had the disadvantage of causing high consumption of the electrode by shortening the discharge period.
The pulse width and the peak value of the discharge current have a close relationship with the processing characteristics and affect the roughness of the processed surface and consumption of the electrode. On the other hand, recently, certain improvements to the processing characteristics have been obtained by passing a pulse current having a special waveform which is different from a rectangular waveform, (e.g., a triangular waveform or a trapezoid waveform). This has been disclosed in the Journal of Denki Kako Gakukai Vol. 3 "Effect of Waveforms in Electrical Discharge Shaping" (First report) by Karafuji, Kinoshita and Fukui wherein it was shown that when the electrical discharge shaping is performed by using a waveform having a slant lift-up line, such as a triangular waveform, the consumption rate of the electrode affected greatly. The circuit of FIG. 1 can be adapted to provide a pulse current having a triangle waveform by including condensers which are inserted in parallel between the collector-emitter of the transistors and reactors which are inserted in series between the power source and the electrode, so as to change the lift-up or lift-down of the rectangular waveform pulse current. However, while somewhat satisfactory, the conventional process utilizes the application of R-C and L-R circuits, which is disadvantageous in that many condensers and reactors are rquired for changing the lift-up or the lift-down in the broad range. Moreover, it is usually hard to form a special waveform having a slant line which is diffierent from triangular or trapezoid waveforms in such an arrangement. In addition, the insertion of a reactor in a circuit is disadvantageous and can cause a spark voltage which in turn could break a trransistor from the viewpoint of the circuit.